The uses of alkali metal-initiated anionic polymeric materials, including diolefin polymers, continue to grow rapidly in such diverse areas as protective paint coverings, wire insulations, structural panels for automobiles, piping and lubricating oil viscosity index improvers. In many of these applications, the stability of the polymer is of paramount importance. Hydrogenation of diolefin polymers greatly improves the stability of these polymers against oxidative, thermal, and ultraviolet degradation. Polymer hydrogenation processes have therefore been studied for many years as a method to prepare novel materials with excellent stability and other desirable properties. Early polymer hydrogenation processes utilized heterogeneous catalysts which were known to be useful for hydrogenation of low molecular weight olefins and aromatics. These catalyst systems included catalysts such as nickel on kieselguhr. A fine catalyst powder was preferred and large amounts of catalysts were required to complete the hydrogenation in a reasonable time. Such processes were only partially successful, since the reaction requires the diffusion of the polymer molecules into the pores of the catalyst, where the active nickel metal is present. This is a slow process when hydrogenating polymers.
Discovery of Group VIII metal, especially cobalt carboxylate and nickel 2-ethyl-1-hexanoate/triethyl aluminum, hydrogenation catalyst systems enabled rapid hydrogenation of polymers. These processes utilize the catalyst as a colloidal suspension in polymer containing solutions. This type of catalyst is referred to as a homogeneous catalyst. Such a process has been used for a number of years to prepare hydrogenated isoprene-styrene block copolymers that are used as viscosity index improvers in premium motor oils. U.S. Pat. No. 3,554,991 describes an exemplary process. Besides cobalt and nickel, Group VIII metals in general will function as the active metal in these systems, and in particular, iron, and palladium are known to be acceptable.
Pore diffusion is not a limitation when homogeneous catalysts are utilized. The hydrogenation process is rapid and complete in a matter of minutes. However, removal of the catalyst from the polymer product is necessary because metals, particularly nickel, which remain with the polymer catalyze degradation of the polymer product. Furthermore, the alkali metal residue, usually lithium, can also cause problems, especially color problems, in the polymer. The removal of the alkali metal initiator and catalyst from the polymer solution is commonly accomplished by the addition of an acidic aqueous solution and air to oxidize the nickel to a divalent state. The nickel and aluminum salts are soluble in the aqueous phase and can then be removed from the hydrogenated polymer solution by separation of the aqueous phase.
Alternative methods to remove hydrogenation catalyst residues from solutions of hydrogenated polymers include treatment with dicarboxylic acid and an oxidant, as disclosed in U.S. Pat. No. 4,595,749; treatment with an amine compound wherein the amine is either a chloride salt or a diamine having an alkyl group of 1 to 12 carbon atoms as disclosed by U.S. Pat. No. 4,098,991; and treatment with a non-aqueous acid followed by neutralization with an anhydrous base and filtration, as to disclosed by U.S. Pat. No. 4,028,485. These processes involve contacting the polymer solution with compounds that contaminate the polymer. Further process steps can be required to remove these contaminants. U.S. Pat. Nos. 4,278,506 and 4,471,099 describe processes to remove such contaminants from hydrogenated polymer solutions. Some of these catalyst removal systems are undesirable because the processes require relatively expensive metallurgy due to the corrosive nature of the compounds. Many also require the consumption of a continuous stream of reactants and produce sludge containing the catalyst and residues of the treatment chemicals.
The above-described processes have the disadvantage that they require process steps such as settling and filtration which separate the catalyst residue. Steps like settling and filtration can be quite expensive in terms of capital investment and time. Thus there is a need for a method which produces stable polymer with good color and which does not require these expensive process steps. The present invention provides a method for producing stable polymer with good color which involves neutralizing the alkali metal initiator rather than removing the catalyst residue from the polymer.